It's more work to buy a bigger size and then cut to your needs. I have often found the smaller sizes on sale at Joann's online; with a free shipping coupon, I often stock up on the smaller sizes that way. I don't use Warm 'n' Natural, though, as the drape is stiffer than I like. I stock up on Mountain Mist Blue Ribbon batting (a very traditional all-cotton batting) in the sizes I need when it's on sale. As long as you have room to store the batting, it can be just as economical doing it this way as investing in larger sizes and cutting them down.
A lot depends on how much money you have available to invest in batting. If you need a small size and only the large size is on sale, and you don't mind the extra work of cutting the batitng down to size and storing the remainder, this can be a good option.